Visible-light laser light sources with monochromaticity high enough to output W-class of high power with three colors such as RGB have broader color reproducibility. Therefore the laser light sources are important for realizing large-screen displays with low power consumption, and compact and brighter illumination apparatuses.
Red-light and blue-light high-power laser light sources has been realized using semiconductor lasers. However it has not been practical to realize a green-light high power laser light source because there are difficulties in practically obtaining optimal materials to fabricate the green-light high power laser light source as a semiconductor laser. Therefore it has been an attractive method to use infrared light as fundamental wave, for example, from a semiconductor laser, fiber laser, or another device to generate green light as the second harmonic wave of the infrared light with a wavelength conversion element including nonlinear optical crystals.
However, speckle noise occurs if a laser is used as a light source of an image display apparatus. For example, light from various regions on the screen overlaps to form an image on the retina of the viewer viewing a virtual image formed by the projector. Meanwhile light beams passing through different regions on the screen overlap in a complex phase relationship interferes because laser light is highly coherent. Such interference results in interference patterns which become unintended optical intensity distributions (speckle noise) to degrade quality of the displayed image.
Speckle noise is a common issue among image display apparatuses, measurement apparatuses, exposure apparatuses and alike in which coherent laser light is used as a light source. Therefore there have been various attempts to reduce speckle noise so far.
For example, Patent Document 1 proposes use of a rotating diffusion plate. According to the disclosed configuration, a diffusion plate operable to rotate at high speed is positioned in an optical path of laser light emitted from a light source. The diffusion plate is rotated at high speed to move the interference pattern generated by the laser light on the screen, so that the interference patterns are averaged to reduce the speckle noise.
In deed, according to the aforementioned configuration, the interference patterns do not disappear but the speckle noise looks disappeared because of the superposition of mutually uncorrelated different interference patterns.
However, in this case, it is also difficult to make speckle noise disappear completely. In particular, prominent speckle noise remains in green light generated by a wavelength conversion element. This results from limited wavelength to be converted to green light in the wavelength conversion element, which in turn leads to narrower wavelength range and more coherent than the red and blue colors from a semiconductor laser.
FIG. 2 shows a schematic configuration of a conventionally used general laser light source 200 conventionally used to generate fundamental wave 203 with a fundamental wave light source 201 and make it incident on a wavelength conversion element 202. The wavelength conversion element 202 converts the fundamental wave light 203 generated in the fundamental wave light source 201 into the second harmonic wave 204. Nonlinear optical crystals, which are lithium niobate-based or lithium tantalite-based, are broadly used as a wavelength conversion element because such optical crystals have larger nonlinear optical constants and are capable of achieving more efficient wavelength conversion. A quasi-phase matched wavelength conversion element in which the polarization direction is reversed with a constant period (Λ) may be formed by using these nonlinear optical crystals to achieve more efficient wavelength conversion.
However, such high wavelength conversion efficiency has not been achieved for all wavelengths of the fundamental wave because the wavelength conversion efficiency depends on the wavelength of the fundamental wave. If the interaction length between the wavelength conversion element 202 and the infrared light as the fundamental wave (in the example of FIG. 2, the total length (L) of the wavelength conversion element 202 is the interaction length) is shortened, wavelength allowance range (range of wavelengths which may be more efficiently wavelength-converted) becomes broader whereas the wavelength conversion efficiency (the efficiency of wavelength conversion of the fundamental wave in the wavelength allowance range) goes down. In short, there is a tradeoff between the wavelength allowance range and the wavelength conversion efficiency.
For this issue, Non-patent Document 1 indicates a method for forming different polarization reversal regions in period in a wavelength conversion element to achieve both the broader wavelength allowance range and more efficient wavelength conversion. As shown in FIG. 3, a wavelength conversion laser light source 300 according to this method comprises a wavelength conversion element 301. The wavelength conversion element 301 comprises polarization reversal regions A1, A2, . . . , An of which polarization reversal periods and lengths are set to Λ1, Λ2, . . . , Λn and L1, L2, . . . , Ln, respectively. It should be noted that the polarization reversal period Λi (i=1, 2, . . . , n) of each region is expressed by Λi=Λ1+ΔΛ(i−1). Therefore the polarization reversal periods of the polarization reversal regions monotonically increase or decrease from the surface onto which the fundamental wave is incident to the surface from which the wavelength-converted light is emitted.
Conversion from infrared light into green light, which is the second harmonic wave of the infrared light, using a lithium niobate-based wavelength conversion element is exemplified. The fundamental wave light source is an infrared light source configured to simultaneously emit a broader wavelength range of light from 1060 nm to 1064 nm in wavelength, as indicated by the spectral distribution of FIG. 17.
The optimum polarization reversal period of the lithium niobate-based wavelength conversion element to convert fundamental wave of wavelength 1060 nm into light of wavelength 530 nm as the second harmonic wave is 6.91 μm whereas the optimum polarization reversal period for conversion from fundamental wave of wavelength 1064 nm into light of wavelength 532 nm is 6.95 μm.
Thus by making Λ1=6.91 μm and Λn=6.95 μm, and adjusting the optimum ΔΛ and length of each polarization reversal region in accordance with the total length of the element, the wavelength conversion element may achieve conversion of fundamental waves at all wavelengths from 1060 nm to 1064 nm into the second harmonic waves, respectively.
In FIG. 3, the same symbols are assigned to the same constituent elements in FIG. 2. Similarly, in this Description hereinafter, the same symbols are assigned to the same constituent elements, of which descriptions may be omitted.
Patent Document 1: Japanese Patent Application Laid-open No. 2007-233371
Non-patent Document 1: Journal of Lightwave Technology, Vol. 26, No. 3, Feb. 1, 2008